Group 56 works for me (at least that's what I've been using all these years) and is still be available from some manufacturers, at least here in the U.S. My favorite battery these days is from Autozone- the Duralast. Autozone Duralast batteries are built by Johnson controls and have more plates per square inch than any battery made. They also put anti-sulfating tablets in the battery so it will not sulfate which leads to shorted or dead cells. I've had great luck with these and use them all my cars now for years. I think they come with a 60 month warranty. At around $60, probably the best deal you'll ever find on a car battery. On one of my cars, the crazy dealers get almost $300 for a crummy battery (one year warranty) and install, go figure. The "Duralast Gold" is a bit more but offers 700 Cold and 875 Cranking amps, plenty for any British car.

I'm not too keen on the Interstate batteries (although I know many people think they are just fine)- I've had numerous problems with those.

I just bought a Group 56 Duralast battery from Autozone and I it does not fit my GT. It is just a bit too wide and will not fit between the back of the batter compartment and the piece of sheet metal that comes down from the inner fender. It is about 3/8 of an inch too wide.

I've got a Duralast 34-DL which fits perfectly size-wise but the positive terminal is on the right. It's the one that was in the car when I bought it.

I researched all the Duralast batteries at Autozone and they don't carry that size with a left pos terminal. The closest are: 86-DL & 56-DL both of which are smaller than the 34-DL.

With the 34-DL the positive cable does reach if you orient the terminals on the firewall side. But it makes it a little scary checking the oil because your hand is dangerously close to the positive terminal while your groin is against the side of the car.

I'm running a Dura Last 36R-DL. It may have required some tweaking of the battery box, I don't remember for sure. It is a nice fit though, the height just clearing the spring hook for the right hand hood balance spring. The previous battery was the same size, but a different brand.

I'll start out, as I just pulled the old battery from my JH.
It was an Interstate MT-42, which is, strangely enough, group size 42.It never actually fit right, so don't even consider getting a size 42. At the time, I had stolen it from one of my other cars and got it to fit by adding 3/4" blocks under it. That got it to clear the wheel well diagonal bit.